A group of metals known as shape memory alloys exhibit a property that when a member formed from such a metal is deformed while below a martensite finished temperature and then is heated to above an austenite temperature, the member returns to the shape existing before the deformation. A well-known shape memory alloy is 55-Nitinol, which is an alloy of nickel and titanium. The use of Nitinol in a heat engine is described in Ginell et al., Nitinol Heat Engines for Low-Grade Thermal Energy Conversion, Mechanical Engineering (May 1979), pp. 28-33. Other applications of shape memory alloys have been disclosed in issued patents. For example, U.S. Pat. No. 4,700,541 issued to Gabriel et al. discloses an electrically-controlled shape memory alloy actuator wherein a wire made of shape memory alloy has its ends constrained against movement and is caused to rotate by the selective application of voltages to different sections of the wire.
U.S. Pat. No. 4,010,455 issued to Stange discloses a thermally-powered rotary actuator that is used for positioning a rotatable shaft in first and second positions disposed 180.degree. apart. The actuator incorporates heat extensible springs formed from 55-Nitinol to apply clockwise or counterclockwise torque to a shaft upon the selective heating of one or the other of the springs.
A mechanical actuator employing Nitinol is also disclosed in U.S. Pat. No. 4,553,393 issued to Ruoff. The actuator of Ruoff is an electro-mechanical servo control system wherein a combination of parallel elements formed from Nitinol may be selectively, electrically heated under digital control to regulate the degree of actuating force provided by the Nitinol actuators.
U.S. Pat. No. 4,665,334 issued to Jamieson describes the use of a shape memory alloy, such as Nitinol, in an actuator to produce rotary motion in the manner of a stepper motor. The memory metal element is employed to impart a drive motion to a spring clutch that is positioned about a shaft. The alternate heating and cooling of the memory metal element causes the spring clutch to tighten and loosen, respectively, to rotate the shaft through a small angle. The memory actuator is selectively, electrically heated.
While the use of shape memory alloys, such as Nitinol, to provide rotary actuators is known, the prior art devices have primarily relied upon electricity to heat the Nitinol above the austenite temperature. In addition to the actuator, a source of electricity is required as well as a control system for selectively applying the electricity to the Nitinol elements. These requirements complicate the construction of the actuator and cause it to be more expensive to manufacture and less reliable to operate. In addition, if an actuator is provided for a single use, the requirement for an electrical control system may make the actuator unnecessarily expensive. Moreover, if a Nitinol actuator is used in a spacecraft or missile to release latches or to provide rotary power for mechanisms therein, the actuator must be lightweight, dependable, and operate with a high degree of precision.